DVDThread: Remove s_dvd_thread_done_working and fix race condition

s_dvd_thread_done_working makes the logic more complicated,
and degasus pointed out a race condition that can happen if
the CPU thread calls WaitForIdle right in between the DVD
thread executing done_working.Set() and done_working.Reset()
while there is work left to do. To avoid this, let's just get
rid of s_dvd_thread_done_working. It's a relic from the old
DVDThread design. Thanks to the last few commits, WaitUntilIdle
only gets called rarely (disc change and savestate), so it's
not a problem if WaitUntilIdle ends up being slower.

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